Acid bath Global ocean acidification caused by acid rain occurred just days after a massive asteroid slammed into the Earth 66 million years ago, new research suggests.

The rapid acidification could explain why surface-dwelling organisms such as ammonites and carbon-secreting plankton were wiped out, while some deep ocean dwellers as well as freshwater species such as crocodiles survived one of the largest mass extinction events in history.

Most scientists believe the Cretaceous-Palaeogene mass extinction event, which wiped out 75 per cent of all life on Earth including the non-avian dinosaurs, was triggered by the impact of a 10-kilometre wide asteroid off Mexico's Yucatan Peninsula.

However, questions about why most species were wiped out while some others survived have remained an area of intense debate among scientists.

The impact, which created the 180-kilometre wide Chicxulub crater, covers an area of seabed that contains anhydrite, a sulphur-rich rock.

While it had been hypothesised that acid rain could have caused the extinction patterns, previous work suggested that sulphur dioxide would have stayed in the atmosphere for months — much too long to cause rapid acidification.

But now Japanese researchers led by Sohsuke Ohno of the Chiba Institute in Japan have examined the composition of the vapour cloud generated by the impact.

Their research, published in the journal Nature Geoscience suggests far more sulphur trioxide than sulphur dioxide was released into the atmosphere.

"Our experiments suggest that the Chicxulub impact released a huge quantity of sulphur trioxide into the atmosphere, where it would have rapidly combined with water vapour to form sulphuric acid aerosol particles," the authors write.

Ohno and colleagues used lasers to fire impactors into anhydrite test samples at velocities of 13 to 25 kilometres per second, which are similar to the speeds expected in an asteroid impact.

Sulphur trioxide reacts quickly with atmospheric water vapour to form sulphuric acid aerosols.

These aerosols would stick to heavier silicate debris particles ejected high into the atmosphere by the impact, and would have fallen back to the surface within two days, much faster than previously thought.

Eureka moment

Professor Mike Archer of the University of New South Wales says the extremely intense acid rainfall and dramatic acidification of global marine surface waters explains the disappearance of many species during this period including ammonites and carbonate secreting plankton.

"It's solved a number of mysteries that have always bothered me," says Archer, who was not involved in the research.

"Their work shows the asteroid could have done all the damage that we suspected it did."

Archer says the relatively moderate rates of extinction in freshwater species could be due to the acid-neutralising effects of the mineral larnite.

"So the crocodiles and many of the terrestrial aquatic organisms would have been able to survive because the acid impact wouldn't have been as bad as it was in the ocean," says Archer.

Intense acidification also explains the so called fern spike when ferns suddenly dominated, right after the impact event.

"The answer might be that ferns are one of the most tolerant plants for dealing with acid," says Archer.

"So when you've got this period of sulphuric acid rain, ferns are more likely to survive than many of the other plants."